State

State Management in Effect

Overview

Effect provides functional mutable state primitives:

• Ref - Basic mutable reference
• SynchronizedRef - Ref with effectful updates
• SubscriptionRef - Ref with change notifications

All are fiber-safe and work correctly with concurrent access.

Ref - Basic Mutable Reference

Creating and Using Refs

import { Effect, Ref } from "effect";

const program = Effect.gen(function* () {
  const counter = yield* Ref.make(0);

  const current = yield* Ref.get(counter);

  yield* Ref.set(counter, 10);

  yield* Ref.update(counter, (n) => n + 1);

  const old = yield* Ref.getAndSet(counter, 0);

  const newValue = yield* Ref.updateAndGet(counter, (n) => n + 5);

  const [oldVal, result] = yield* Ref.modify(counter, (n) => [n, n * 2]);
});

Atomic Operations

const atomicIncrement = Effect.gen(function* () {
  const counter = yield* Ref.make(0);

  yield* Effect.all(
    [Ref.update(counter, (n) => n + 1), Ref.update(counter, (n) => n + 1), Ref.update(counter, (n) => n + 1)],
    { concurrency: "unbounded" },
  );

  return yield* Ref.get(counter);
});

Ref in Services

const CounterService = Effect.gen(function* () {
  const ref = yield* Ref.make(0);

  return {
    increment: Ref.update(ref, (n) => n + 1),
    decrement: Ref.update(ref, (n) => n - 1),
    get: Ref.get(ref),
    reset: Ref.set(ref, 0),
  };
});

const CounterLive = Layer.effect(Counter, CounterService);

SynchronizedRef - Effectful Updates

For updates that require running effects:

import { Effect, SynchronizedRef } from "effect";

const program = Effect.gen(function* () {
  const ref = yield* SynchronizedRef.make({ count: 0, lastUpdated: Date.now() });

  yield* SynchronizedRef.updateEffect(ref, (state) =>
    Effect.gen(function* () {
      yield* Effect.log("Updating state");
      return {
        count: state.count + 1,
        lastUpdated: Date.now(),
      };
    }),
  );

  const result = yield* SynchronizedRef.modifyEffect(ref, (state) =>
    Effect.gen(function* () {
      const newCount = state.count + 1;
      yield* sendMetric("counter", newCount);
      return [newCount, { ...state, count: newCount }];
    }),
  );
});

When to Use SynchronizedRef

• Updates require API calls
• Updates require logging/metrics
• Updates depend on external state
• Updates need error handling

// Cache with async refresh
const cache = yield * SynchronizedRef.make<Data | null>(null);

const refreshCache = SynchronizedRef.updateEffect(cache, () => Effect.tryPromise(() => fetchLatestData()));

SubscriptionRef - Reactive State

For state that needs to notify subscribers:

import { Effect, SubscriptionRef, Stream } from "effect";

const program = Effect.gen(function* () {
  const ref = yield* SubscriptionRef.make(0);

  const changes = yield* SubscriptionRef.changes(ref);

  yield* Effect.fork(Stream.runForEach(changes, (value) => Effect.log(`Value changed to: ${value}`)));

  yield* SubscriptionRef.set(ref, 1);
  yield* SubscriptionRef.update(ref, (n) => n + 1);
  yield* SubscriptionRef.set(ref, 10);
});

Reactive Patterns

const configRef = yield * SubscriptionRef.make(initialConfig);

const subscriber1 = Effect.fork(
  Stream.runForEach(SubscriptionRef.changes(configRef), (config) => updateService1(config)),
);

const subscriber2 = Effect.fork(
  Stream.runForEach(SubscriptionRef.changes(configRef), (config) => updateService2(config)),
);

yield * SubscriptionRef.set(configRef, newConfig);

Comparison

Common Patterns

Counter Service

class Counter extends Context.Tag("Counter")<
  Counter,
  {
    readonly increment: Effect.Effect<number>;
    readonly decrement: Effect.Effect<number>;
    readonly get: Effect.Effect<number>;
  }
>() {}

const CounterLive = Layer.effect(
  Counter,
  Effect.gen(function* () {
    const ref = yield* Ref.make(0);
    return {
      increment: Ref.updateAndGet(ref, (n) => n + 1),
      decrement: Ref.updateAndGet(ref, (n) => n - 1),
      get: Ref.get(ref),
    };
  }),
);

State Machine

type State = "idle" | "loading" | "success" | "error";

const stateMachine = Effect.gen(function* () {
  const state = yield* Ref.make<State>("idle");

  const transition = (from: State, to: State) =>
    Ref.modify(state, (current) => (current === from ? [true, to] : [false, current]));

  return {
    state: Ref.get(state),
    startLoading: transition("idle", "loading"),
    succeed: transition("loading", "success"),
    fail: transition("loading", "error"),
    reset: Ref.set(state, "idle"),
  };
});

Accumulator

const accumulator = Effect.gen(function* () {
  const items = yield* Ref.make<Array<Item>>([]);

  return {
    add: (item: Item) => Ref.update(items, (arr) => [...arr, item]),
    getAll: Ref.get(items),
    clear: Ref.set(items, []),
    count: Effect.map(Ref.get(items), (arr) => arr.length),
  };
});

Best Practices

1. Use Ref for simple state - Basic counters, flags, accumulators
2. Use SynchronizedRef for async updates - When updates need effects
3. Use SubscriptionRef for reactive patterns - When others need notifications
4. Keep state minimal - Don't store derived data
5. Prefer immutable updates - Return new objects, don't mutate

Additional Resources

For comprehensive state management documentation, consult ${CLAUDE_PLUGIN_ROOT}/references/llms-full.txt.

Search for these sections:

• "Ref" for basic mutable references
• "SynchronizedRef" for effectful updates
• "SubscriptionRef" for reactive state

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